Carburizing is an effective method of increasing the surface hardness of low carbon, unalloyed, or low carbon, low alloy steels. Typically, steel articles are placed in an atmosphere containing carbon in an amount greater than the base carbon content of the steel and heated to a temperature above the austenite transformation temperature of the steel. After the desired amount of carbon has been diffused into the article, hardness is induced by quenching.
Gas carburizing is efficient, controllable, and one of the most widely used methods of supplying the carbon to the surface of the steel.
Conventional carburizing and hardening processes typically try to avoid the formation of case carbides. The general aim is to produce an essentially carbide free martensitic structure in the carburized and hardened case. Silicon effects surface carbide formation and, by limiting the amount of silicon in a carburizing steel, as taught in the present invention and as set forth in U.S. Pat. No. 4,921,025 "Carburized Low Silicon Steel Article and Process" which issued May 1, 1990 to Sheryl A. Tipton et al. and is assigned to Caterpillar Inc.,surface carbides may be easily formed.
Conventional gas carburizing processes, as discussed above, generally attempt to prevent the formation of case carbides by maintaining carbon potential at or slightly above eutectoid carbon content. A nonconventional carburizing process for intentionally forming carbides in the case is described in Canadian Pat. 610,554, "Carburization of Ferrous Alloys", issued Dec. 13, 1960 to Orville E. Cullen. Cullen teaches a method for carburizing low alloy steel by repeatedly carburizing and rapidly cooling the steel article.
More recently, a two stage carburizing process was described in U.S. Pat. No. 4,202,710, issued May 13, 1980, to Takeshi Naito, et al. That process forms spheroidal carbides within a region between 0.1 mm and 0.4 mm below the case surface, but fails to provide a high density of carbides on the outer surface of the carburized case. As a result, articles formed by this teaching must initially wear, or be machined down to the carbide rich zone beginning 0.1 mm below the surface before the enhanced wear and contact fatigue properties of the carbide microstructure, such as pitting and spalling resistance, can be advantageously utilized.
It is well known that lubricated concentrated rolling contacts can fail from surface or subsurface initiated pitting. Sliding contacts can fail from excessive wear, scuffing or seizure. These failure mechanisms are controlled globally by oil film thickness, hertzian contact stresses, and lubrication at asperity contacts. These factors, in conjunction with other factors, determine the distribution of contact stresses near asperities, friction coefficient, and contact flash temperature. All of these factors will influence pitting and scuffing failures.
Contacts in gears usually operate in the region of mixed-film lubrication where the film thickness to roughness ratio, lambda, is less than three. This results in the load being shared between the fluid and the asperity contact. The lubrication behavior in this region is influenced by the overall distributions of lubricant film thickness, pressure, shear stress and flash temperatures within the hertzian contact and the local variation of these quantities around the asperity contacts.
Micro-EHL theory developed by H. S. Cheng and Nadir Patir, "An Average Flow Model for Determining Effects of Three-Dimensional Roughness on Partial Hydrodynamic Lubrication", ASME Vol 100, January 1978, proposes that the ratio of metallic asperity contact to total area depends on the directionality of the surface roughness, or lay of machine grooves. The model is demonstrated and the flow resistance of the lubricant is greater in FIG. 1-B and FIG. 1-C than in FIG. 1-A of Prior Art FIG. 1. The volume of the retained lubricant is increased and therefore the thickness of the oil film is also increased.
Akamatsu et al, "Improvement of Roller Bearing Fatigue Life by Surface Roughness Modification", SAE paper No. 910958, 42nd Earthmoving Industry Conference, Peoria, Ill. April 1991, taught how to expand on Cheng's theory and produce rolling element bearings with a superfinished surface exhibiting small (10 micron) random pits. The depth of the pits are about (1 micron). The resultant surface possesses no directional characteristics (isotropy). The isotropic texture resulted in 2.0 to 13 times longer life than normal bearings.
Sliding, inherent in gear contacts, shifts the peak value of the hertzian shear stress to the surface. This combination of rolling and sliding can initiate fatigue cracks at the surface that will result in pit formation.
A high percentage of carbides produced at the surface of a component result in greater high temperature strength, resistance to wear, and increased pitting life.
In addition to the differences of the above cited references and the instant invention, none of the references teach or suggest isotropically exposed carbides on the surface which have an elevation greater than material adjacent the carbides of the carburized surface or coating said surface with a coating material having a hardness greater than about 1200 Kg/mm for the purpose of enhancing oil film thickness, friction coefficient, contact stresses and contact temperatures which influence pitting and scuffing performance.